schemagen: deduplicate enum constants that collide after sanitization

Distinct JSON enum values can sanitize to the same C++ identifier
(e.g. "foo-bar" and "foo_bar" both become `foo_bar`), producing an
invalid `enum class` with duplicate constants.

Add `unique_enum_identifiers()` which appends a numeric suffix to
later collisions while preserving enum declaration order, so the
index-to-JSON-value mapping used by the generated parser stays intact.

Also add contrib/schemagen/test_schemagen.py and wire the schemagen
Python tests plus the existing example.schema.json/test_gen.cpp example
into ctest via CMakeLists.txt.

Closes #4

contrib/schemagen/test_schemagen.py

@@ -0,0 +1,53 @@
+#!/usr/bin/env python3
+"""Tests for weaseljson_schemagen.py."""
+
+import json
+import os
+import subprocess
+import sys
+import tempfile
+import unittest
+
+SCRIPT = os.path.join(os.path.dirname(__file__), "weaseljson_schemagen.py")
+
+
+class SchemagenEnumTest(unittest.TestCase):
+    def run_schemagen(self, schema, args=None):
+        """Run schemagen on a schema dict. Returns (returncode, stdout, stderr)."""
+        with tempfile.NamedTemporaryFile("w", suffix=".json", delete=False) as fp:
+            json.dump(schema, fp)
+            schema_path = fp.name
+        try:
+            cmd = [sys.executable, SCRIPT, schema_path]
+            if args:
+                cmd.extend(args)
+            result = subprocess.run(cmd, capture_output=True, text=True, check=False)
+            return result.returncode, result.stdout, result.stderr
+        finally:
+            os.unlink(schema_path)
+
+    def test_colliding_enum_values_deduplicate(self):
+        schema = {
+            "type": "object",
+            "properties": {"role": {"enum": ["foo-bar", "foo_bar"]}},
+        }
+        rc, stdout, stderr = self.run_schemagen(schema)
+        self.assertEqual(rc, 0, msg=stderr)
+        self.assertIn("enum class Role : int { foo_bar, foo_bar_1 };", stdout)
+        self.assertIn(
+            'static constexpr const char *Role_names[] = { "foo-bar", "foo_bar" };',
+            stdout,
+        )
+
+    def test_distinct_enum_values_generate(self):
+        schema = {
+            "type": "object",
+            "properties": {"role": {"enum": ["admin", "user", "guest"]}},
+        }
+        rc, stdout, stderr = self.run_schemagen(schema)
+        self.assertEqual(rc, 0, msg=stderr)
+        self.assertIn("enum class Role : int { admin, user, guest };", stdout)
+
+
+if __name__ == "__main__":
+    unittest.main()